In the latest constitutional clash between the White House and what it views as an obstructionist Congress, the court was warned by Obama's solicitor general, Donald Verrilli, that it risks upsetting Washington's “stable equilibrium” by removing the “safety valve” of so-called recess appointments.

But most of the nine supreme court justices who heard argument in a test case on Monday appeared to take a dim view of the government's position, suggesting the historic exemption from Senate confirmation requirements during recess was no longer suitable in a era when Congressmen are able to return more readily to Washington in emergencies.

“Most modern presidents have used [recess appointments] not to deal with congressional absence but congressional intransigence,” said Justice Elena Kagan. “This is not the horse and buggy era. There is no such thing as congressional absence here. What we are dealing with is a historical relic.”

The test case was brought by a bottling company, Noel Canning, against the National Labor Relations Board, whose Obama-appointed members ruled against the company in a dispute with its unions.

In this instance, the appointment of NLRB members during the Senate's Christmas break was further complicated by a growing practice in Congress to hold "pro-forma" sessions, in which a token senator bangs the gavel during holiday periods, to prevent triggering a formal recess. The White House has largely ignored such symbolic measures for the purposes of defining whether recess appointments were allowed or not, but Kagan and other justices said they believed it was up to the Senate to decide whether it was in recess or not.

During technical arguments that drew on 18th-century British parliamentary definitions of the word "recess", a number of justices appeared to agree that US constitutional language only allows presidents to circumvent the confirmation process in very limited circumstances involving recent vacancies.

But there was sympathy for the administration's argument that language in the constitution was ambiguous and had already been deemed to have evolved by many previous presidents.

Verrilli said defeat in this case would “repudiate hundreds if not thousands of appointments going back to George Washington” and could even open such appointees' decisions to retroactive legal challenge. “It may be true as a matter of raw power that the Senate is able to sit on appointments for months or years at a time but that is a hundred miles from what the framers [of the constitution] intended,” Verilli added.

But the conservative justice Anton Scalia scoffed at the idea of revisiting past appointments and asked if the administration's argument simply meant “if you ignore the constitution often enough, its meaning changes”.

Republicans argue that the constitution was intended to make it difficult for presidents to push through unpopular appointments.

“This is the bottom of a slippery slope,” said Miguel Estrada, representing Senate Republican leader Mitch McConnell in support of the Noel Canning case. “It's a complete abuse of the process used for no other purpose than to overcome Senate objection.

“What the framer's had in mind was that you have to act nice,” he added. “In a country of 300 million people it is always possible to come up with another person who is acceptable to the Senate.”

Nevertheless, the case is seen as having important ramifications for the wider balance of power between the executive and legislative branches and could be vital if Democrats lose control of the Senate in November's mid-term elections.

“We are confident that the president's legal right to make recess appointments will be upheld,” said Obama's spokesman, Jay Carney, after the morning hearing.